1. Field of the Invention
The present invention relates to the field of high pressure enclosures, and maintaining a seal on such an enclosure.
2. Description of the Related Art
High-pressure fluid pumps are used in various industrial applications. For example, a high-pressure pump may be used to provide a pressure stream of water for cleaning and surface preparation of a wide variety of objects, from machine parts to ship hulls.
High-pressure pumps may also be used to provide a stream of pressurized water for water jet cutting. In such an application, a pump pressurizes a stream of water, which flows through an orifice to form a high-pressure fluid jet. If desired, the fluid stream may be mixed with abrasive particles to form an abrasive water jet, which is then forced through a nozzle against a surface of material to be cut. Such cutting systems are commonly used to cut a wide variety of materials, including glass, ceramic, stone and various metals, such as brass, aluminum, and stainless steel, to name a few. A single pump may be used to provide pressurized fluid to one or several tools.
In another application, high-pressure fluid pumps are used for isostatic pressurization, used in many industrial applications, including processing of foods, manufacture of machine parts, and densification of various components and materials.
A detailed description of the operation of a high-pressure pump may be found in U.S. Pat. No. 6,092,370, issued on Jul. 25, 2000, in the name of Tremoulet, Jr. et al., which patent is incorporated herein by reference in its entirety.
FIG. 1 illustrates a simplified cut away of a pump head of a typical high-pressure fluid pump. The fluid pump 100 includes a cylinder 102 and plunger 104. A valve body 114 is located at the end of the cylinder 102, and incorporates inlet and outlet check valves (not shown in detail). The valve body 114 is held in place against the cylinder by an end cap 106. Tie rods 108 pass through apertures in the end cap 106 to engage the pump body (not shown). Torque nuts 110 and thrust washers 112 engage upper ends of the tie rods 108 to draw the end cap 106 tightly against the cylinder 102, capturing the valve body 114 therebetween. A plunger or piston 104 is positioned within the cylinder to pressurize fluid in the cylinder.
An annular seal or gasket 116 is positioned between the valve body 114 and the end of the cylinder 102 to create a static seal configured to prevent fluid from passing between the valve body 114 and the cylinder 102. The gasket 116 may be made from a polymeric material or from another material that is softer than the materials used to make the valve body 114 and the cylinder 102, even including a metal gasket.
Another type of static seal, in which the valve body is biased directly against the cylinder, is described in U.S. patent application Ser. No. 10/038,507, entitled “Components, Systems and Methods for Forming a Gasketless Seal Between Like Metal Components in an Ultrahigh Pressure System,” which is assigned to Flow International Corporation and is incorporated herein by reference in its entirety.
Fluid pumps of the type described herein are used to generate fluid pressures of between 30,000 and 100,000 psi. Because of the very high pressure generated within the cylinder 102 during a pressurizing stroke of the plunger 104, one of the most common problems in pumps of this type is failure of the static seal 116. In such a failure, fluid is forced between the valve body 114 and the cylinder walls 102, to escape the pump. Such a failure results in a reduction in the overall pressure generated by the pump 100, and damage to the pump itself, as fluid, passing at high pressures through unintended pathways, causes fatigue and erosion.
A very high degree of force, pressing the valve body 114 against the cylinder 102, is required to reduce the occurrence of such failures of the static seal 116. In a pump of the type illustrated in FIG. 1, this force is achieved by extremely high torque on the tie rod nuts 110 on each of four tie rods 108. To achieve the necessary force, torque in the range of 700 foot-pounds on each of the tie rod nuts 110 may be required. However, torque at this high level creates several significant complications, apart from the high degree of effort required to install and remove the nuts 110. First, as torque is applied to a tie rod nut 110, friction between the nut and the tie rod 108 places rotational stress on the tie rod 108. As torque on the tie rod nut 110 increases, rotational force on the tie rod 108, caused by friction, begins to twist the tie rod 108. When the appropriate torque is achieved on the tie rod nut 110, and the torquing force is removed, the tie rod 108 exerts a reverse rotational force on the tie rod nut 110 and the thrust washer 112. This same reverse rotational force is exerted by each of the tie rods 108 on each of the tie rod nuts 110 and thrust washers 112. As a result, a general rotational load is placed on the end cap 106. Part of this rotational load is transferred from the end cap 106 to the cylinder 102, placing undesirable forces on the pump 100, and even causing the end cap 106 and cylinder 102 to twist one or two degrees.
Additionally, at high torque loads, such as those discussed above, a large part of the total force generated by the high degree of torque placed on the tie rod nut 110 is expended in overcoming friction between the nut 110, the washer 112, and the tie rod 108. This part of the total force generated is lost to friction, and is not ultimately expressed as additional tensile load on the tie rod 108. As torque on the tie rod nut 110 increases, the total percentage of force lost to friction rises in a nonlinear fashion. Worse, this rise is unpredictable, very difficult to measure, and may vary, at the high torque loads required, by as much as 40% from one tie rod 108 to another. As a result, the four tie rods 108 of a pump cylinder 102, each having a tie rod nut 110 set at 700 foot-pounds of torque, may have vastly different tensile loads. These different loads can cause the end cap 106 to tilt, or to press with more force on one side of the cylinder 102 than the other, again causing accelerated failure of the static seal 116.
One solution to the problems caused by high torque on the tie rod nuts 110 is the use of super nuts as illustrated in FIG. 2, which shows a portion of an end cap 106 where a tie rod 108 protrudes. The super nut 130 is threaded onto the tie rod 108, and tightened to a much lower torque load of between 20 and 50 foot-pounds of torque. The super nut 130 includes a plurality of apertures into which jack bolts 132 are threaded. Each super nut has between 12 and 16 jack bolts. The jack bolts 132 pass through the super nut 130 to make contact with the thrust washer 112. Each jack bolt 132 presses against the thrust washer 112, pulling up on the super nut 130 and the tie rod 108. While each jack bolt 132 is applied with a modest degree of torque, the total force exerted by the jack bolts 132 of each of the four super nuts 130 is sufficient to maintain the necessary pressure on the end cap 106. Because the torque on each of the jack bolts is much lower, the percentage of the force generated lost to friction is also much lower. Additionally, because each super nut 130 has as many as 16 jack bolts, variations in force lost to friction by each jack bolt 130 will average out, resulting in a generally equal force on each tie rod 108.
There are, however, drawbacks to the use of super nuts 130. One drawback is the additional time required for installation or removal of the super nuts 130. When installing or removing the super nuts 130, torque on each of the jack bolts 132 must be applied or released gradually and cyclically, meaning that each of the jack bolts 132 on each of the super nuts 130 must be loosened or tightened by a very small amount, in turn, and repeatedly, until all of the bolts 132 of all the super nuts 130 have been fully loosened or fully tightened. This process is very time consuming, and can add two or more hours to the time required for removal and replacement of the end cap during servicing. Additionally, super nuts 130 and jack bolts 132 are subject to wear and fatigue, such that over time and repeated removal and re-installation, changes will occur in their response to tensile load and friction. As a result, combining new parts with old parts on a single pump head can result in uneven load conditions, again resulting in accelerated wear on the pump itself.
A second solution to the problems associated with high torque on the tie rod nuts 110 is described with reference to FIG. 3, and in more detail in U.S. Pat. No. 5,037,276, issued to Tremoulet, Jr. FIG. 3 illustrates a portion of a pump 134 having an output chamber 137 located between the valve body 114 and the end cap 106. An outlet port 162 admits pressurized fluid from the cylinder 102 to the outlet chamber 137 at the end of a pressurizing stroke of the piston 104, pressurizing the output chamber 137. Pressurized fluid exits the output chamber 137 via an output line 133, after which the fluid is channeled to an output manifold, or directly to an output tool. Meanwhile, the output chamber 137 remains pressurized at, or near, the maximum pressure achieved in the cylinder 102.
The end cap 106 applies downward pressure on the valve body 114, pressing the valve body against the cylinder 102, with static seal 116 therebetween. When the pump 134 begins operation, the output chamber 137 is charged to a pressure approaching that of the pressure within the cylinder 102. The pressurized fluid within the output chamber 137 exerts an upward force on the end cap 106, which loads the tie rods. Meanwhile, downward force on an upper surface 136 of the valve body 114 is equal to, or greater than, upward force on the lower surface 135 of the valve body, thus providing sufficient force to maintain the static seal 116.
One drawback to this solution is the need for an additional static seal 117, which must also withstand the high pressure generated within the cylinder 102. A more serious problem, however, is the fact that the tie rods 108 are unloaded every time the pump is turned off and the pressure within the output chamber is allowed to bleed away. This situation creates excessive stress on the tie rods, as they are repeatedly loaded and unloaded each time the pump 134 is turned on and off.
Another solution is proposed in U.S. Pat. No. 5,302,087, issued to Pacht, and described with reference to FIG. 4. A pump 210 includes a pressure housing 212, located between the end cap 106, and the valve body 114. The pressure housing comprises a liquid pressure chamber 214, with a pressure transmitting piston 216 located therein. Pressure from the outlet port 162 is transmitted to the liquid pressure chamber 214 via a flow line 136, a control valve 140, an additional flow line 138, and a flow path 142, which delivers pressurized fluid to the liquid pressure chamber 214.
When the pump 210 begins operation, the control valve 140 is opened, permitting pressurized fluid to pass through the control valve along the flow lines 136, 138, to the liquid pressure chamber 214, pressurizing the chamber 214 to a pressure approximately equal to the pressure produced within the cylinder 102. The pressure transmitting piston 216 is pressed upward against the end cap 106, loading the tie rods 108 and exerting pressure on the static seals 116. Once the liquid pressure chamber 214 is pressurized, the control valve 140 is closed, trapping the pressure within the pressure chamber 214. In this way, the tie rods remain loaded, even during periods when the pump 210 is not in operation.
Nevertheless, this solution is not without drawbacks. For example, the external compression lines 136, 138 are subject to failure due to the high pressure produced by the pump 210. Additionally, seals within the liquid pressure chamber 214 must withstand the high pressure produced by the pump 210.